Optical low-coherence reflectometry (OLCR) is used for example for analyzing inhomogeneities in optical waveguides and optical devices. In this method light is transmitted down the optical fibre and light resulting from the interaction with an inhomogeneity in the optical fibre is back-scattered. The light is split into two arms, a sample arm and a reference arm. When the optical pathlength in the sample arm matches time delay in the reference arm coherent interference occurs and the distance the light has travelled in the sample arm may be determined.
Most known devices, use broadband light sources eg. superluminescent diodes, with a short coherence time, and they need a scanning mirror to record the depth resolved backscattered signal. In other systems a tunable laser is used as the light source, whereby, instead of moving the mirror, the wavelength of the laser can be varied to record the backscattered signal. This principle is discussed in Haberland, U.H.P. et al., “Chirp Optical Coherence Tomography of Layered Scattering Media” as well as in U.S. Pat. No. 5,956,355 (Swanson et al.). The method is often referred to as coherent optical frequency modulated continuous wave (FMCW) reflectometry.
OLCR can be extended through the use of polarized light. The light field towards the reference and sample is then polarized. After combining the light field reflected from the reference and the sample, the combined light field is split up again into two new light fields with perpendicular polarization states. Through this method the birefringent properties of the sample can be investigated in addition to the information obtainable with ordinary OLCR adding to the systems ability to discriminate between certain types of materials within the sample. This method also applies to OCT often referred to as polarization sensitive OCT (PS-OCT), as well as coherent optical FMCW reflectometry.
Optical low-coherence reflectometry is also used in the imaging of 2-dimensional and 3 dimensional structures, eg. biological tissues, in this respect often referred to as optical coherence tomography (OCT). OCT can be used to perform high-resolution cross-sectional in vivo and in situ imaging of microstructures, such as in transparent as well as non-transparent biological tissue or other absorbing and/or random media in generel. There are a number of applications for OCT, such as non-invasive medical diagnostic tests also called optical biopsies. For example cancer tissue and healthy tissue can be distinguished by means of different optical properties. Coherent optical FMCW reflectometry also applies to the above-mentioned cases.
In order to optimize optical low-coherence reflectometry measurements and imaging various suggestions to increase signal-to-noise ratio (SNR) have been discussed in the art.
U.S. Pat. No. 5,291,267 (Sorin et al.) discloses optical reflectometry for analyzing inhomogeneities in optical fibres. In U.S. Pat. No. 5,291,267 amplification of the light reflected from the optical fibre is conducted. In particular U.S. Pat. No. 5,291,267 suggests to use the light source as an amplifier in order to save costs.
WO 99/46557 (Optical Biopsies Technologies) discusses SNR in a system wherein a reference beam is routed into a long arm of an interferometer by a polarizing beamsplitter. In general the reference suggest to include an attenuator in the reference arm to increase SNR. In a balanced setup the reference on the other hand suggests to increase the power of the reference arm in order to increase SNR.
In “Unbalanced versus balanced operation in an optical coherence tomography system” Podoleanu, A. G., Vol. 39, No. 1, Applied optics, discussed various methods of increasing SNR in unbalanced and balanced systems, respectively. Reduction of power in the reference arm was suggested as well as reduction of fibre end reflections to increase the SNR.
Optical low-coherence tomography reflectometry and coherent optical FMCW reflectometry obtain the same information about the sample being investigated, and, in this respect, they may be considered similar.
The present invention relates to an optimisation coherent optical FMCW reflectometry whereby an increase of the SNR is obtained leading to a better result of the measurements, in particular in relation to penetration depth of the system, so that the penetration depth increases, when the SNR increases.